Extra dexterity and much time needed for a luthier to do so the expensive vs. inexpensive would be a vague response. You will need a quote and obtain a price and then ask yourself if that price is worth it when you get a real dollar amount. Of course this amount will be an addition to the cost of a new fret job.
I 've wondered how hard it would be to add nibs after the fact. You would have to heat up the neck/plastic, remove it, then maybe use a special file to grind down the fret ends to accomodate new nibs. Then glue back on some plastic and cut out the in between bits. It would make sense on an expensive vintage guitar so maybe the same thing applies to the 70s/80s MIJ LPs since they are essentially classified as vintage now.
I think I have seen a few guitars on YJ where they have done this.
Erick Coleman's restoring a 1950s Les Paul, and returning the bound fretboard to its original condition. With small files and elbow grease, he shows how to create the fret-end "binding nibs" for that distinctive Gibson look.
If you are doing thin vintage style binding where the nib is partly down the fret end then you also have to bevel the fret and nibs together.